Process for flash spinning an integral web of polypropylene plexifilaments

ABSTRACT

A WEB OF ENTANGLED PLEXIFILAMENTS OF ISOTACTIC POLY PROPYLENE IS OBTAINED BY FLASH SPINNING A SOLUTION OF THE POLYMER THROUGH TWO OR MORE CLOSELY-SPACED SPINNERET ORIFICES. DEPENDING UPON ARRANGEMENT OF THE ORIFICES, THE WEB MAY BE A YARN OR TOW OF SHAPED CROSS-SECTION, OR IT MAY BE A RIBBON OR SHEET PRODUCT. THE SOLVENT MAY BE 1,1,2-TRICHLORO - 1,2,2 - TRIFLUOROETHANE, TRICHLOROFLUOROMETHANE OR A MIXTURE THEREOF.

R. WOODELL I 3,564,088 PROCESS FOR FLASH'SPINNING INTEGRAL WEB 0FPOLYPROPYLENE PLEX LAMEN'I'S Sheets-Sheet 1 'Fiied-Sept., 5, 1969 vINVENTOR f \\\\j r S RUDOLPH woooeu.

'ZI BY I ATTORNEY 16, 1971 R WQQDE LL 3,564,088 PROCESS FOR FLASHSPINNING AN INTEGRAL POL - WEB OF YPROPYLENE PLEXIFILAMENTS Filed Sept.:5, 1969 r 2- Sheets-Sheet 2 Y 500 IOOO I |5OO 3; ;4 3 3 |4 E X X/ /J ifQ. j L L 21o I j TEMPERATURE C ATTORNEY United States Patent O US. Cl.264205 7 Claims ABSTRACT OF THE DISCLOSURE A Web of entangledplexifilaments of isotactic polypropylene is obtained by flash spinninga solution of the polymer through two or more closely-spaced spinneretorifices. Depending upon arrangement of the orifices, the web may be ayarn or tow of shaped cross-section, or it may be a ribbon or sheetproduct. The solvent may be 1,1,2-trichloro 1,2,2 trifluoroethane,trichlorofiuoromethane or a mixture thereof.

RELATED APPLICATIONS This application is a continuation-in-part of US.patent application Ser. No. 768,617, filed Oct. 15, 1968, now US. Pat.3,467,744, which is in turn a continuation-inpart of US. Ser. No.506,304, filed Nov. 4, 1965, now abandoned.

BACKGROUND OF THE INVENTION In the US. Pat. 3,081,519 of Blades andWhite a method is described for preparing a fibrillated web orpleXifilament by flash spinning. In this process a polymer solution at atemperature above the boiling point of the solvent and at a pressure atleast autogenous is extruded into a medium of lower temperature andsubstantially lower pressure. The sudden boiling which occurs at thispoint causes either microcellular structures or fibrillated networks toform. The fibrillated materials tend to be formed when the pressurechanges are most severe, or when more dilute solutions are used. Underthese circumstances the vaporizing liquid within the extrudate formsbubbles, breaks through confining walls, and cools the extrudate causingsolid polymer to form therefrom. The resulting multifibrous yarn-likestrand has an internal fine structure or morphology characterized as athree-dimensional integral plexus consisting of a multitude ofessentially longitudinally extended, interconnecting, random-length,fibrous elements, referred to as film-fibrils.

Previous investigation with linear polyethylene has shown that undercertain conditions the spinning solution forms a cloudy dispersionwhich, if allowed to stand without adequate agitation, settles into twodistinct layers, one layer being rich in polymer and the other layerbeing lean in polymer. This phenomenon is described in Anderson andRomano.U.S. Pat. 3,227,794, issued Jan. 4, 1966.

Further. problems were encountered in developing an eflicient processfor spinning all species of isotactic polypropylene by the process ofAnderson and Romano. These problems were overcome by the improvedtechnique 3,564,088 Patented Feb. 16, 1971 claimed in US. Pat.3,467,744. In essence, the improvement consisted of using a specificsolvent and maintaining temperatures and pressure considerably abovethose specified by Anderson and Romano. In the course of developing thisimprovement, I found that several strands of the highly fibrillatedproducts can be spun simultaneously from several closely-spaced orificesto prepare an integral cohesive web.

SUMMARY OF THE INVENTION The purpose of the present invention is toprovide an efiicient process for preparing an integral cohesive web fromseveral continuous strands of flash-spun fibrillated isotacticpolypropylene. The cohesive web may be in the form of continuous yarn ortow having a shaped crosssection, or may be a ribbon, or a sheetproduct. The strands are aligned principally in the lengthwise directionof the web and the fibrils of adjacent strands are entangled, therebyproviding a single web which cannot be separated into constituentstrands without tearing. The process of this invention involves forminga homogeneous single-phase polymer solution at a temperature which isabove the critical temperature of the lowest boiling solventconstituent, and at a pressure which is above the two-liquid-phasepressure boundary for the solution, then passing the solution into apressure let-down zone for lowering the pressure of the solution toabout 10 to 400 p.s.i. below the two-liquid-phase pressure boundary forthe solution, and finally, discharging the solution through two or moreclosely spaced spinneret orifices of restricted size to an area ofsubstantially atmospheric pressure and ambient temperature. The orificesare of about equal size and are between 0.01 and 0.05 inch in diameter.The distance between any orifice and the next adjacent orifice measuredcenter-to-center is 5 to 30 times the diameter of the orifices. Thesolution which is extruded through the orifices comprises 4 to 20% byweight isotactic polypropylene and the solvent is achlorofluoroaliphatic material with cirtical temperature preferablybetween and 220 C.

FIG. 1 is a transverse cross-sectional view of an integral web oftrilobal cross-sectional shape configuration (enlarged about 4- to10-fold).

FIGS. 2a, 2b, 2c are diagrams of spinneret faces showing arrangement oforifices for spinning webs of trilobal, tetralobal, or ribboncross-sections, respectivley.

FIG. 3 is a longitudinal cross-sectional view of a solution supply tube,a letdown chamber, and a spinneret.

FIG. 4 is a graph of pressure and temperature conditions for solutionsof 1,1,2-trichloro-1,2,2-trifluoroethane and isotactic polypropylene,showing the location of twoliquid-phase pressure boundaries.

FIGS. 5a, 5b, and 5c are detail drawings of the spinneret portion ofFIG. 3 when 4 orifices are present.

FIGS. 6a, 6b, and 6c are detail drawings of another spinneret portionusable with the apparatus of FIG. 3 and having 3 orifices arranged in atriangular pattern.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The process of the inventionprovides a variety of products when process conditions are controlledwithin the scope of the invention. For example, one may attain acontinuous strand 1 having a trilobal shape such as shown in the FIG. 1.The entire strand is comprised of film-fibril elements 2 as described inBlades & White US. 3,081,519. The film-fibrils are interconnected bypoints along and across the lobes 3 of the strand forming an integralnetwork in three dimensions. The individual lobes are tied to each othersimply by entanglement. This entanglement occurs only when a high degreeof fibrillation is obtained during spinning and when the orificedimensions and spacings are properly controlled.

Other shapes such as tetralobal may be obtained by the process of theinvention. In addition, tapes may be spun by the use of a number ofholes arranged in a straight line, and a sheet product may be obtainedby use of a large number of holes. All of the coherent integral webproducts are characterized by the same extremely thin film-fibrils. Thefilm-fibrils are less than 4 microns thick and are molecularly orientedin the general direction of the longitudinal axis of the web. The degreeof fibrillation is high, and the products have a surface area greaterthan 2 m. g.

The solvents which are useful in the invention are chlorofluoroaliphaticcompounds or mixtures of such compounds. The following solvents areincluded:

1,1,2-trichloro-1,2,2-trifluoroethane (Freon-1 13) boiling point: 47.6C.

critical temperature: 214 C.

critical pressure: 480 lbs./in. (gage) trichlorofiuoromethane (Freon1 l)boiling point: 24 C.

critical temperature: l98200 C.

critical pressure: 620-640 lbs/in. (gage) mixtures of Freonl1 andFreon-1l3.

The preferred mixture contains equal weights of Freon-11 and Freonl13.When using solvent mixtures, it is important that the solutiontemperature upstream of the let-down chamber be above the criticaltemperature of the lowest boiling constituent, i.e. above 198-200 C. forthe Freonl1/Freon-l13 mixture. The pressure must be above thetwo-liquid-phase pressure boundary for the solution. A technique forlocating the boundary is disclosed in Us. Pat. 3,467,744. The statedpressures and temperatures for the spinning solution upstream of thelet-down orifice must be maintained throughout the spinning operation inorder to obtain continuously a high degree of fibrillation throughoutthe length of the integral product. In addition, in order to obtain anentangled integral web with tenacity above 0.3 gram per denier, theprocess of the present invention requires passing the solution which isat a pressure above the two-liquid-phase pressure through an orificeinto a let-down chamber having pressure about 30 to 400 p.s.i. below thetwo-liquid-phase pressure. The extent to which the pressure in thelet-down chamber should fall below the two-liquid-phase pressureboundary varies with the solvent used. The optimum pressure in thelet-down chamber can readily be determined for a given solvent system.If the pressure in the let-down chamber falls too far below thetWo-liquid-phase pressure boundary the product will be discontinuous,foamy particles, called fly. If the pressure does not fall suflicientlyfar below the two-liquid-phase pressure boundary the fibrils will bepoorly separated and little or no entangling will take place, so theproduct will not be integral.

The size of the final orifices and the distance between the orifices arecritical in determining the degree of entanglement betweenplexifilaments from any two adjacent orifices. Round orifices having adiameter between 0.01 and 0.05 inch are preferred. The center-to-centerdistance between any orifice and the adjacent orifice should be 5-30times the diameter of the orifices. In the examples, the orificediameters are indicated by Q and the distance between orifices(center-to-center) is indicated by the symbol P (see FIGS. 2a, 2b, and2c). When the ratio of P/Q is greater than 30, the flash-spunplexifilaments fail to entangle under the influence of the evaporatingsolvent. It should be understood that with the particular P conditionsspecified, the solvent evaporates at a very high rate and creates muchturbulence around the spinneret. This tends to cause entanglement ofadjacent filaments when two strands are close enough. On the other hand,when the P/ Q ratio is less than 5, the integral strand seems to loseits shape definition and become similar to a single strand inappearance. The shape of the integral web is largely determined by theplacement of the spinneret orifices relative to one another. Atetralobal strand is obtained from the spin orifice arrangement shown inFIG. 2a, a trilobal strand from 212, and a tape from 20.

The temperature and pressure combinations for operation of the processwith Freon-l 13 are shown in FIG. 4. In the figure, curve A is the vaporpressure curve for the solvent. Line B is the critical temperature ofthe solvent, and Line C is the critical pressure, this being thepressure of the solvent under autogenous pressure at the criticaltemperature. The two-liquid-phase pressure boundary at variousconcentrations is indicated by curves H, J, and L. In FIG. 4, theseboundaries H, I, and L apply to solutions containing 13, 10, and 4% byweight, respectively, of polypropylene in Freon-113. Temperature andpressure conditions to the right and below line H give a singlephasesolution for a 13% solution. Temperature and pressure conditions to theleft and above line H give cloudy solutions having tWo-liquid-phases,one rich in polymer and the other lean with respect to polymer. Furtherdetails on the location of the two-liquid-phase pressure boundary can beobtained by reference to my parent applications.

For 13% solutions, for example, the particular operating conditions inFIG. 4 which give a sufiicient degree of fibrillation for entanglementof the isotactic polypropylene plexifilaments are those in which thesolution upstream of the first orifice is at a temperature above thecritical temperature shown by line B and at a pressure above thetwo-liquid-phase pressure boundary shown by line H. The process of theinvention further requires that the pressure he reduced by passage ofthe solution through a pre-flash chamber of the type shown, for example,in FIG. 3. In the pre-fiash chamber the pressure must be reduced so thatit falls to the left and above line H, but not below the criticalpressure. Finally, the solution passes through the spinneret orificeinto the surrounding atmosphere. At this point, the flash-spunplexifilaments from adjacent orifices combine to give an integralproduct.

The particular spinning conditions which are preferred give a highdegree of fibrillation and avoid fusion of film-fibrils. No additionalair-jets are needed for entangling the adjacent lines and no dryingprocess is needed, since the evaporation is adiabatic and since adequateheat is applied upstream of the pre-fiash chamber to evaporate thepreferred solvents.

In determining the minimum operable solution pressures and temperaturesfor spinning solutions containing mixtures of solvents, one uses thecritical temperature of the lowest boiling constituent as a minimum andthe critical pressure of the highest boiling constituent as a minimum.For this purpose, one should ignore small quantities of extraneousmaterials in the solvent. If less than 10% of such material is present,it is not included as a constituen In preparing the solution the polymerand solvent are mixed by any of a number of known methods. For example,powdered isotactic polypropylene may be blended with liquid1,1,Z-trichloro-1,2,2-trifluoroethane at room temperature to form adispersion. The resulting dispersion (slurry) may then be heated withstirring in the vessel which is to serve as a supply reservoir forspinning, or it may be continuously pumped through a heat exchanger to aspinneret or spinning cell. In either case the solution must bedelivered to the let-down chamber at a temperature above the criticaltemperature and at a pressure greater than the two-liquid-phase boundarypressure. The additional pressure can be created by pressurizing with aninert gas such as nitrogen. Such an inert gas should preferably not bemixed with the solution but rather should be present as a force pressingagainst it. Alternatively, it can be generated (1) by mechanical meanssuch as one or more pumps, or (2) by heating the blend to the desiredtemperature in a vessel with a volume that is small enough to enable thesolution to generate sufficient pressure to eliminate any gas phase,above the solution at the desired temperature. The blend should containbetween 4 and 20% polymer and 96 to 80% solvent. These percentages, aswell as others referred to in the description which follows, are on aweight basis.

The polymer used in the solution should have a melt flow rate between0.09 and 10.0, the units as used throughout being in g./ min. The methodfor determining melt flow rate is ASTM Method 123 8T, Condition L forpolypropylene.

The polymer used for preparing the solution is not necessarily composedof 100% propylene. The polymer should contain at least 85% of propylene,but may have as much as by weight of units derived from otherethylenically unsaturated monomers such as isobutylene, vinyl acetate,methyl methacrylate, or mixtures such as efhylene/octened. The termisotactic polypropylene as used herein refers to such polymerscontaining a high proportion, e.g. over 80% by weight, of isotacticmacromolecules. A further description thereof is given by Natta et al.in U.S. Pat. 3,166,608.

In the examples which follow, a batch process is used for preparingsolutions. For this purpose it is important to determine the amount ofpolymer and solvent which is needed to provide in the autoclave ahomogeneous single-phase solution at a desired operating pressure andtemperature. In other words, sufficient solution must be present in theautoclave to prevent the formation of a solvent vapor phase. Theapproximate amount of material may be calculated from the density of thesolution at the desired spinning temperature and pressure. The solventis added while the autoclave is under vacuum. The autoclave is thenclosed. The agitator is turned on and the autoclave heated as rapidly aspossible while a graph of the temperature and pressure is made duringthe heatup cycle. Excessive pressure (due to minor errors in calculationor inaccurate density values) may be released by bleeding 01f smallportions of the material from the autoclave from time to time.

When the solution is ready for batch flash-spinning, the agitator isstopped and the atmosphere above the solution is pressurized withnitrogen to a level 100 to 200 p.s.i.g. above the autoclave pressure.Stirring is avoided to prevent mixing of the nitrogen gas with thesolution. The nitrogen pressure within the autoclave is maintained atthis level so that no pressure drop will occur during spinning. Thetotal pressure is recorded as the solution pressure and the 100-200p.s.i.g. increment is included as though it were solvent-generated.

Although the use of nitrogen or other inert gas as above described willbe illustrated in the examples which follow, it will be understood thatfor a commercial operation a piston or other mechanical means would bepreferable.

Suitable spinnerets for use with the process of this invention are shownin FIGS. 3, 5a, 5b, 5c, 6a, 6b, and 60. FIG. 3 shows a longitudinalcross-section of a spinneret assembly which is attached to a solutionsupply by means of pipe thread 32. The spinneret assembly comprises acylindrical tube 31 provided with an integral cap 37 and containing aspinneret 24, a let-down chamber insert 33 with a hollow cylindricalcore, a let-down orifice insert 34 having a small orifice 35 and ahollow spacefilling insert 36 to provide adequate support for the otherportions. All of the parts are machined to fit inside the outer tubularportion 31 and are gasketed to provide a pressure-tight system. Onespinneret 11 for use in the spinneret assembly is shown in FIG. I511.The spinneret is held in place by means of shoulder 14. FIG. 5a is across-sectional side view showing a deep slot 10. FIG. 5b is a top viewof the slot potrion of FIG. 5a. Both figures show the four orifices 12aligned in a row at the bottom of the slot. FIG. 5c is an enlargedcross-section of a single orifice 12. The inner end of each orificepassage is countersunk to provide conical taper 13. The land length 15includes only the tiny cylindrical portion of the orifice.

Another spinneret 24 which may be used in the same spinneret assembly isshown in FIG. 6a. The portion of this spinneret which is upstream of theorifices contains a wide bore cylindrical portion 20. Three spinneretorifices 21 are bored in the bottom of the wide cylindrical portion.These are equi-spaced around the axis of the cylinder on circle 22. Inthis case the orifices are not countersunk.

EXAMPLE I A solution of isotactic polypropylene was prepared frompolymer having a melt flow rate of 0.8 g./ 10 min. usingl,1,2-trichloro-1,2,2-trifluoroethane (Freon-11 3) as solvent. Thesolution was flash-spun using the conditions specified in Table l. Thesolution contained 10% polymer by weight and was spun through aspinneret having four orifices as shown in FIGS. 5a, 5b, and 5c. Thecylindrical portion of the let-down chamber for this spinneret assemblywas 0.5 inch in diameter and 3.31 inches long. The slot 10 was 0.7 inchlong (measured transversely) and 0.75 inch deep. The slot width was 0.25inch.

The product was a tape in which the original four strands were barelyvisible, being somewhat denser than the connecting film-fibril web. Thefour strand residues ran side-by-side through the length of the tape.The tape had a tenacity of 1.62 g./denier, elongation of 99% at breakand denier of 56.

TABLE 1 Item No. I

Polymer melt flow rate 0.8 Solution:

Percent polymer 10 Temp. C. 219

Pressure lbs./in. (gage) 1300 Let-down chamber Inlet orifice diameter,inches .018

Pressure lbs/in. (gage) 900 Spinneret:

Extrusion rate, lbs. polymer hour 6 No. of orifices 1 4 Distance P, in..130

Diameter Q, in. .010

Land length, in. .021

EXAMPLE II A solution was prepared containing 8% by weight isotacticpolypropylene (melt flow rate 0.85 g./l0 min.) and 92% by weighttrichlorofiuoromethane (Freon-11). A homogeneous solution was obtainedby heating the solvent and polymer in an autoclave to 2l9222 C. which isabove the critical temperature 198-200 C. Five different spinningconditions were tested as shown in Table 2. In each case the solutionwas spun from the same autoclave through a cylindrical pre-flash chamber0.5 inch in diameter and 3.31 inches long. The round orifice at theinlet to the let-down chamber was 0.031 inch in diameter. The finalorifices at the spinneret face consisted of three holes arranged in anequilateral triangular pattern. These holes were 0.014 inch in diameterand the distance between holes was 0.250 inch (P/Q: 17.9). Thelandlength through spinneret face was 0.020 inch. Since a vessel oflimited size was used, the pressure rose to between 1630 and 1685 poundsper square inch, which is above the twoliquid-phase pressure boundaryfor an 8% solution of this polypropylene in Freon1 l. The products whichwere obtained at various spinning temperatures and pressures areindicated in Table 2. Each of the products had denier between 115 and194, tenacity between 1.7 and 2.7 g.p.d. and elongation at break of 54to 73%. Apparently for item II-D the pressure in the let-down zone wasnot sufliciently far below the two-liquid-phase pressure boundary forthis 8 was between 1290 and 1730 lbs./in. (gage). Pressures in thisrange are above the two-liquid-phase pressure boundary of the mixtureand above the critical pressure of Freon-l13 (480 lbs./in.

The solution was flash extruded through a spinneret 5 solution. similarto the one shown in FIG. 5a, but having 5 orifices TABLE 2 arranged in astraight line and having a conical lead-in instead of the oval slot 10.The narrow portion of the Item cone 1s 0.5 in dlameter and the conewidens as it ap- 11-13 proaches the orifice. The solution was suppliedfrom the Solution temp, O c 220 221 221 222 autoclave at temperaturesand pressures indicated in Table Solution pressure in autoela ,lbs.

(gage) f 1,670 1,660 1,630 1,685 there being four different parts to theexperiment. In ghailebounriaryylbg/mt (gage)l 1,540 1,560 1,560 1,580each case, the solution passed from the autoclave through etown 0 ambere, b (gage) 1,310 1,445 1,340 1,480 an inlet orrfice to a pre flashchamber. The inlet orifice Extrusion rat polymer 1 h 23 19 21 19 was0.036 inch in diameter. The cylindrical portion of Remarks Integral 0)Integral (2) the pre-flash chamber was 0.5 inch in diameter and 3.31

1 Borderline integral. inches in length. 2 Not mtegml- The distancebetween spinneret orifices center-to-center EXAMPLE III Was 0.250 inchfor each part of the experiment. The orifice diameters were each 0.014inch. P/ Q was therefore Solutions were prepared from isotacticpolypropylene 17.9. The land length was 0.020. Under each of thecondihavlpg a melt ew rate of 0.7 g- 1( y h g m tions described in Table4, a bulky tape was obtained. The arnixture containing equal quantitiesby welght of 1,1, five components were thoroughly entangled to provide atflehlol'o 1,2,2 tflfillofoethane and coherent integral web which couldnot be separated again ehlofofllloromethahe ease the Sohl- 25 into thecomponents. When the tape was spread transtion composed y Welght P y eand 90% y versely, a uniform sheet about two inches wlde was obvflelghty Ihlxture- A homogeneous slngle'phase Solo" tained. The edges of thefive original strands were faintly PP- h lbdlvldbal lobes of the WebCould not be P visible and were oriented along the length of the strandditions 1nd1cated1n Table 3. i d l fa hion Thesolutior} epasjscdthroughacylingiricallet-down A sheet or much greater width can beobtained by chamber 0.5 inch in diameter and 3.31 ln g, the spinningthrough a larger number of orifices arranged in orifice at the inlet toth1s chamber b ing .020 to Q lheh a straight line. One may also spinthrough several rows in diameter as indicated in Table 3. The solutionwas f rifi to b i Planar Sheets or webs having extruded throughsplnnelets eaoh havlng three c1osely selected cross-sectional shape orhaving greater density spaced holes arranged in an equilateraltriangular pattern. unif rmity The spinneret of Item III-A had orificesarranged to give TABLE 4 a P/ Q ratio of 7.15. Under the temperature andpressure conditions indicated in Table 3 for Item III-A, this spinneretgave an integral tow which was trilobal in cross-sec- Solution temp., C1 2% 1 Egg 1 1 3 1 st ,.n. e tron. The indlvidual lobes of the web couldnot be sepa- 40 j g rated without tearing the film-fibrils. The tow fromItem lb5./il1. 1,080 1, 000 1,120 1,140 III-A had a denier of 231,tenacity of 2.3 g./denier, and lbs/m 2 950 870 1,015 1,075 elongation69% at break. Extrusionrate, polymer lbs. 43 37 43 Items III-D throughIII-G each gave a tow with trilobal 23 53,; 393 i 7?, $3 3'72cross-section. These products were very bulky. The denier 45 Elon ation,percent at break 71 73 74 70 for these yarns was between 360 and 400,the tenacity 1.33 to 1.61 g./denier, and the elongation 70 to 74%. Whatis cla1med1s: Items IIIB and III-C with P/ Q 35.8 did not give aninte- 1. In the process of flash spinning isotactic polygral cohesiveproduct. proylene plexifilaments by the steps of (a) forming a TABLE 3Item III-A III-B III-C III-D III-E III-F III-G Solution:

Temp, C 210 208 208 209 210 2 212 Pressure, lbs./in. 2, 020 1,650 1,7801,615 1,790 1, J 1,980 Two-liquid-phase pressure boundary, lbs./in.1,120 1,080 1,080 1,100 1,120 1,140 1,160 Inletorifice diam.,in .020 026.026 .026 026 .026 .026 Let-down chamber:

Pressure. 1bs./in. 840 000 1,050 905 955 1,000 1,025 Extrusion rate,lbs. polymer/hr 24 22 22 27 30 31 30 spinneret:

Distance P, in 0.100 0,500 0.500 O. 250 0. 250 0.250 0. 250 Diamet r Q,in 0. 014 0. 014 0.014 0. 014 0. 0145 0.0145 0.0145 Ratio P/Q 7,15 35,835.8 17.3 17.3 17.3 17.3 Land length, in 0.20 .020 .020 .020 .020 .020.020

EXAMPLE IV homogeneous single-phase solution of polypropylene, having amelt flow rate between 0.09 and 10.0 g./ 10 min., in A Solution ofisotaetie P yp py (melt flow rate a chlorofluoroaliphatic solventselected from the group 0.7 g./10 min.) was prepared using a mixture of50% consisting of 1,1,2-trichloro-1,2,2-trifluoroethane, trichloy Weight(boiling Point and 50% rofluoromethane and mixtures thereof having acritical Freo11-11 as Solvent- The Solution temperature between 190 and220 C., bringing it to a tained 10% polypropylene and solvent mixture. Ahomogeneous single phase was obtained by heating a slurry of the statedcomposition in an autoclave to between 207 and 210 C. as indicated inTable 4. Temperatures in this range are above the critical temperatureof Freon-11 (l98200 (3.). The pressure in the autoclave temperatureabove the critical temperature of the lowest boiling component of thesolvent and to a pressure above the two-liquid-phase pressure boundaryfor the solution, the said solution having a concentration of between 4and 20% by weight of the polymer, (b) passing the solution into apressure letdown zone for lowering the pressure of the solution tobetween about 10 and 400 p.s.i. below the two-liquid-phase pressureboundary for the solution, and (c) discharging the solution through aspin neret orifice into an area of substantially atmospheric pressureand ambient temperature to provide a continuous, highly fibrillatedstrand, the improvement which comprises the step (c), discharging thesolution through at least two orifices of about equal diameter betweenabout 0.01 and 0.05 inch, the center-to-center distance between anyorifice and the next adjacent orifice being between about 5 and 30 timesthe orifice diameter, whereby there is formed a single cohesive integralweb composed of a plurality of entangled plexifilamentary strands.

2. Improvement of claim 1 wherein the solvent is 1,1,2-trichl0ro-1,2,2-trifluoroethane.

3. Improvement of claim 1 wherein the solvent is trichlorofluoromethane.

4. Improvement of claim 1 wherein the solvent is a mixture of1,1,2-trichloro-1,2,2-trifluoroethane and trichlorofluoromethane.

5. Improvement of claim 1 wherein the solution is References CitedUNITED STATES PATENTS 3,504,076 3/1970 Lee 264205 JULIUS FROME, PrimaryExaminer H. MINTZ, Assistant Examiner US. Cl. X.R.

